When a semiconductor wafer (hereinafter also referred to as “wafer”) as an object to be processed is washed, there have been conventionally used an ammonia peroxide mixture (SC1) made by adding NH4OH (ammonium hydroxide) and a hydrogen peroxide solution to a deionized water as a solvent, a hydrochloric acid-peroxide mixture (SC2) made by adding hydrochloric acid and a hydrogen peroxide solution to a deionized water as a solvent, and diluted hydrofluoric acid made by diluting hydrofluoric acid with a deionized water.
The ammonia peroxide mixture (SC1) is mainly used for removing particles adhering to a wafer, the hydrochloric acid-peroxide mixture is mainly used for removing metal contaminations from a wafer, and the diluted hydrofluoric acid is mainly used for removing contaminations from a wafer.
In order to measure concentrations of chemical liquids, such as the ammonia peroxide mixture (SC1), the hydrochloric acid-peroxide mixture (SC2), and the diluted hydrofluoric acid, which are contained in process liquids, there are known a method of measuring a conductivity of a process liquid, and a method of measuring a transmittance of light passing through a process liquid (absorbance of the process liquid) (see, JP62-8040A, JP10-154683A, and JP2005-189207A).
However, as described above, the ammonia peroxide mixture (SC1) contains two chemical liquids, i.e., NH4OH and a hydrogen peroxide solution. Similarly, the hydrochloric acid-peroxide mixture (SC2) contains two chemical liquids, i.e., hydrochloric acid and a hydrogen peroxide solution.
Thus, when a conductivity of a process liquid is measured so as to measure concentrations of the chemical liquids, since a conductive effect of one chemical liquid and a conductive effect of the other chemical liquid are mixed with each other, it is difficult to measure exact concentrations of the respective chemical liquids. Meanwhile, when a light transmittance (absorbance) of the process liquid is measured so as to measure concentrations of the chemical liquids, although concentrations of the respective chemical liquids can be exactly measured, a long period is required for the measurement.
When there is used a process liquid containing a chemical liquid of a relatively higher density, and thus a large amount of the chemical liquid is supplied, a concentration of the chemical liquid can be measured by directly measuring a flow rate of the chemical liquid to be supplied. However, when there is used a process liquid containing a chemical liquid of a relatively lower density, since an amount of the chemical liquid to be supplied is small, it is extremely difficult to exactly measure a flow rate of the chemical liquid which is being supplied.